Xeroprinting is a process in which a plate having a permanent or persistent image or pattern of insulating material on a grounded conductive backing is charged, e.g., by corona discharge, to produce a latent image of electrostatic charge on the insulating areas, the image is developed by toning with oppositely charged particles, and the toner image is transferred by electrostatic or other means to another surface such as paper or polymeric film. Since the original image is permanent or persistent, multiple copies can be made with a single exposure by repeating the charging, toning and transfer steps.
Various methods have been devised to prepare electrostatic masters. One method, disclosed in U.S. Pat. No. Schaffert 2,576,041 and Yang 3,515,584, involves coating an electrically conductive backing plate with a hardenable material, exposing to light through a negative to harden exposed areas, and washing out unexposed areas. This method has the disadvantage of requiring a processing step (wash-out) between exposure and toning.
Another method, known as chemography, involves coating a conductive substrate with an insulating layer containing a chemical sensitizer which increases in conductivity when exposed to light. Imagewise exposure creates a pattern of exposed and unexposed zones differing in conductivity. Charging creates an image of electrostatic charge on the unexposed, lower conductivity areas. Toning the entire layer with oppositely charged particles produces a toner image on the unexposed areas which can be transferred to another surface in the xeroprinting process. Patents which relate to this method include Ebert Canadian Nos. 618,521 and 618,594; Cassiers et al. U.S. Pat. No. 3,113,022; Hepher U.S. 3,311,471 and Etoh et al. U.S. Pat. No. 4,111,692.
The Canadian patents also describe a corollary, opposite embodiment in which imagewise exposure to light results in decreased conductivity in exposed areas, and charging creates a latent electrostatic image on the exposed areas. An example of this appears in the Canadian No. 618,594 in which the sensitizer was chloroacetic acid.
It is known that photopolymerizable systems based on ethylenically unsaturated monomers decrease in conductivity upon polymerization. This property has been used in electrographic application, but apparently not in xeroprinting. For example, Ingersoll U.S. Pat. No. 3,286,025 discloses imagewise exposing a photopolymerizable layer comprising a polymeric binder, an ethylenically unsaturated monomer and, optionally, a photoinitiator. The exposure creates polymerized areas of reduced conductivity, which can be corona charged on a conductive support and toned to form a toner image. The patent does not disclose transferring the toner image to another support; it does state that multiple copies can be made, but the procedure involves washing out nonimage areas with a solvent, then placing the plate on an offset press and printing by lithographic techniques.
Although the Ingersoll patent states that either electron beam or actinic radiation can be used for exposure all examples use electron beam. The patent discloses various methods of reading the image obtained by exposure; Example XIII is the only one which illustrates toning. Experiments with a system similar to that used in Example XIII indicate that such a system, when exposed with actinic radiation, is not polymerized sufficiently to retain charge long enough for toning. These experiments are reported in Example 9 below. The photoinitiator used, an anthraquinone, was not as effective as the HABI initiators used in this invention.
HABI-initiated photopolymerizable systems have been used in an electrostatic imaging application. Research Disclosure, item number 15514, pages 9-10, March 1977, discloses a process involving briefly exposing a photopolymerizable system imagewise to increase electrical resistance without substantial polymerization, corona charging to produce an electrostatic image, and toning to develop the image. The system exemplified was the system of Chang U.S. Pat. No. 3,549,367, Example 2. This system contained a HABI initiator, 2-(o-chlorophenyl)-4,5-bis(methoxyphenyl)imidazolyl dimer; a binder, poly(methylmethacrylate/methacrylic acid); a monomer trimethylolpropane triacrylate; and a co-initiator, 4,4'-bisdimethylamino-benzophenone. There is no indication in the Research Disclosure item that the system could be used in an electrostatic master for xeroprinting, i.e., that the toner image could be transferred. Instead, it is disclosed that the toned element can be re-exposed for a longer time to polymerize previously unexposed areas, then the previously exposed areas can be washed out to yield a printing plate.
The Chang et al. patent is one of several which disclose HABI-initiated photopolymerizable systems for use in non-electrostatic imaging processes. Others are identified below. Films based on such systems are sold commercially for non-electrostatic imaging applications.
The only known prior disclosure of the use of a photopolymer system to produce a master for xeroprinting is in Bickmore U.S. Pat. No. 3,518,081 (U.S. Pat. No. Re. 29357). In this patent, a photopolymer is imagewise exposed to polymerize exposed areas, then the entire surface is rubbed with another material to produce a triboelectric charge pattern corresponding to the image. This pattern can be toned to produce a toner image which can be transferred to another surface. This process does not depend on differing conductivity of exposed and unexposed areas, but on differences in triboelectric charging characteristics. The patent states that one attempt to charge an imagewise exposed photopolymer layer by corona discharge, followed by toning, produced no visible image.